The present invention relates to a lubricating apparatus for a ball screw, and more particularly to a lubricating thread capable of supplying a lubricant to a ball screw for a long time.
A ball screw is a mechanism for converting rotational motion into linear motion. In the ball screw, a plurality of balls are interposed between a screw shaft having a thread groove on an outer periphery thereof and a ball nut having a thread groove on an inner periphery thereof opposed to the screw shaft thread groove. The ball nut is moved axially by rotating the screw shaft.
A lubricant such as a grease is normally applied to a portion between the thread groove and the ball, thereby preventing the thread groove and the ball from abrading and heating because of the friction between the thread groove and the ball. However, the effectiveness of the grease in reducing the friction is quickly diminished by such factors as temperature, moisture, foreign material and abrasion, such that it is necessary to periodically add additional lubricant. Therefore, apparatuses for supplying additional lubricant have been designed.
However, such supplying apparatuses are typically expensive. Moreover, because it is necessary to provide a piping for supplying oil to the apparatus, additional costs are incurred. Further, these apparatuses are inconvenient in that it is necessary to inspect and maintain the supplying apparatus.
By comparison, it has also been suggested to dispose a polymer member containing a lubricant between the screw shaft of the ball screw and the ball nut. The polymer member is designed so that the lubricant gradually oozes out of the polymer member over a long period of time. Thus, the thread groove and the ball remain lubricated as a consequence of the lubricant oozing out of the polymer member containing the lubricant even after the lubricating function of the grease initially applied to the thread groove is diminished. Accordingly, such a device is substantially maintenance-free.
There are still problems with using the polymer member described above. By way of illustration, FIG. 15 is a cross sectional view which shows a ball screw using a polymer member containing a lubricant as described above. In FIG. 15, a cylindrical ball nut 2 is disposed around a screw shaft 1. A polymer member 10 containing a lubricant is disposed between an inner peripheral surface 5a of an annular recess portion 5 formed on an end surface of the ball nut and an outer peripheral surface of the screw shaft 1. The polymer member 10 acts as a seal to seal grease within the ball nut 2.
A garter spring 15 is fitted into an outer peripheral surface lob of the polymer member 10. With the garter spring 15 radially pressing the polymer member 10 into the thread groove 1b of the screw shaft 1 at a fixed pressure, suitable contact with the screw shaft 1 is maintained and good lubrication is maintained even when the inner peripheral surface of the polymer member 10 is abraded.
FIG. 16 is a cross sectional view of the ball screw of FIG. 15 taken along a line XVI--XVI. FIG. 17 is an enlarged cross sectional view of the ball screw of FIG. 15 taken along a line XVII--XVII.
In FIG. 16, the polymer member 10 includes an upper half portion 10c and a lower half portion 10d, each formed in a substantially half cylindrical shape. In the upper half portion 10c and the lower half portion 10d, all of the inner peripheral portions are in close contact with the outer peripheral portion of the screw shaft 1. A gap A is formed between the upper half portion 10c and the lower half portion 10d.
Comparing FIG. 16 with FIG. 15, it will be recognized that a convex portion 10a of the polymer member 10 is disposed in such a manner as to be in close contact with the outer peripheral portion of the thread groove 1b of the screw shaft 1. As shown in FIGS. 16 and 17, when the convex portion 10a of the polymer member 10 is in close contact with the thread groove 1b and the screw shaft 1 oscillates relative to the cylindrical ball nut 2, the lubricant attached to an inner portion of the thread groove 1b is scraped out to the stroke end of the screw shaft 1 by a sliding convex portion 10a of the polymer member 10. In fact, as shown in FIG. 16, when an end portion (a portion facing to the gap A) of the convex portion 10a comes in close contact with the thread groove 1b without any gap, there occurs a so-called scraper effect, that is, the end portion scrapes out the lubricant attached to the thread groove 1b to the gap A. The lubricant scraped out to the gap A is hardly used for lubricating.
Thus, the amount of the lubricant essentially supplied to the thread groove is decreased because of the shape of the polymer member 10 containing the lubricant. In an application wherein the ball screw supports a large load, not enough lubricant will remain to lubricate the ball screw, such that there is a risk of abnormal abrasion and early breakage of the thread groove and the ball.